1. Field of the Invention
The present invention relates to an absorption belt capable of highly accurately attracting and holding an object, more particularly to an absorption belt suitable for the transfer of a printing media in a copying machine, a laser beam printer, a facsimile, an ink jet printer or a multifunction machine having functions of these apparatuses, an image forming apparatus with such an absorption belt and a method for producing such an absorption belt.
2. Description of the Related Art
Conventionally, in a copying machine, a laser beam printer, a facsimile, an ink jet printer or a multifunction machine having functions of these apparatuses, an absorption belt capable of attracting and holding a printing medium has been used as a means for transferring the printing medium such as paper, OHP film or the like. As an example of such absorption belt, a single-layer or a multi-layer resin belt having a properly set resistance value is known. In transferring the printing medium by this kind of absorption belt, both the surface and the back of the absorption belt is electrically charged so that the printing medium can be attracted thereto.
However, in the case of the above-mentioned image forming apparatus, the increase in the transfer speed of the printing medium sometimes has caused the printing medium to run afloat above the resin belt due to the resistance of the air that affects the flow of the printing medium while being transferred. Such floating of the printing medium can be considered to result from (1) the effect of the physical external force such as the vibration of the belt or the like and (2) the temporary decrease in the surface electric charge and the resulting inability of the belt for maintaining the surface electric charge at a necessary level owing to the temporary decrease in the surface resistance of the belt due to the cause such as the drop of the ink onto the belt in the case of the ink jet printer or the like.
A known technique for resolving such a problem is an absorption belt including a pair of pectinate electrodes disposed on the surface or inside the resin belt (Refer to Japanese Patent Application Publication No.57-058872 (1982)). In the absorption belt, the pectinate electrodes are disposed opposing and alternately to each other leaving predetermined interposing gaps. In order for letting the absorption belt exert a necessary attractive force on any object to be attracted, each of the paired opposing pectinate electrodes needs to have either positive or negative electric charge applied thereto. The object can be locally charged to be attracted by the absorption belt owing to the effect of the static electric charge. Further, in general, the absorption belt having a pair of pectinate electrodes is respectively provided with feeding terminals at both ends thereof so that each of the pectinate electrodes is fed with either a positive voltage or a negative voltage. In another case, the feeding terminals are provided on the side of one of longitudinal edges of the belt in order to reduce the width of the belt (Refer to Japanese Patent Application Laid-Open No.2000-095376).
The statically charged absorption belt, having the pectinate electrodes as described above, is capable of attracting the object more stably and strongly compared with the resin belt designed to attract the object by electrically charging the resin belt. Thus, this kind of static absorption belt is advantageous when used as the printing medium transfer means for the full-line type ink jet printing apparatus employing the line-type print head having a plurality of discharge ports arranged along the direction of the width of the printing medium, and, in this way, the printing operation at higher speeds can be made possible (Refer to Japanese Patent Application Laid-Open No.11-151842 (1999)).
On the other hand, known as the conventional method for producing the absorption belt are (1) a thermal extrusion molding process as a representative inflation process and (2) a casting process for obtaining desired belt by first applying a predetermined quantity of molten resin on both the outer surface and the inner surface of a cylindrical mold and then by removing the applied resin after undergoing the solvent removing process.
Further, known as another method for producing the absorption belt is (3) a process for manufacturing the belt from a thermoplastic sheet placed between a core member and a cylindrical member (Refer to Japanese Patent Application Laid-Open No. 8-187773 (1996)). In the method (3), first the thermoplastic sheet is wound on the core member until the both ends of the thermoplastic sheet properly overlap with each other, and then the core member is inserted into the cylindrical member whose thermal expansion coefficient is lower than that of the core member. Then, these members are heated to reduce the gap between the core member and the cylindrical member, and the both ends of the thermoplastic sheet are fused to join with each other.
In manufacturing the absorption belt having the pectinate electrodes placed on the surface or inside thereof, among the applicable methods there are a process wherein the desired absorption belt is manufactured by combining the sheet containing the preformed electrode pattern with any one of the method (1) through (3), and the other process wherein after the body of the belt is manufactured by one of the methods (1) through (3), the electrode pattern is formed in the belt by one of various processes.
However, the absorption belts produced by the conventional processes have some drawbacks as are described in the following. For instance, in the conventional absorption belt, a relatively large leakage current is apt to occur between the adjacent electrodes having the polarities opposite to each other, thereby causing the deterioration of the absorption belt and the resulting negative effect on the reliability thereof. Further, the conventional absorption belt has a shortcoming such that a large power consumption is needed for obtaining a sufficient attractive force, and it has been considered that such a large power consumption have room for further reduction of the total power consumption by the apparatus.
In addition, for the further compactness of the image forming apparatus, it is also necessary to reduce the size (width) of the absorption belt, but, even if both the feeding terminal for the application of the positive voltage and the feeding terminal for the application of the negative voltage arranged only on the side of one of the longitudinal edges like in one of the cases of the conventional absorption belts, it is necessary to provide an insulators for preventing the feeding terminals for different polarities from coming into contact with one another. Thus, in order to reduce the size of the absorption belt, it is necessary to adopt some appropriate measures for satisfying such requirement.
On the other hand, there are problems as are described in the following concerning the manufacturing process of the absorption belt. For instance, the absorption belt manufacturing process characterized by first manufacturing the resin belt by one of the methods (1) through (3) followed by the process for forming the electrode pattern on the resin belt is more costly than the belt manufacturing process characterized by using the sheet with preformed electrode pattern followed by the manufacture of the absorption belt by one of the methods (1) through (3).
Further, when the above described method (1), i.e., the thermal fusion molding process (extrusion molding process and inflation process) is employed, it is necessary to take up the absorption belt which is molded continuously, and the belt is apt to be depressed or bent while being taken up, and such depression or bend of the belt adversely affect the object of the transfer, i.e., the printing medium. In order to resolve such a problem, it becomes necessary to provide a long molding line and a long space therefore that ends up with a higher manufacturing cost. Further, the casting method corresponding to the above-mentioned method (2) involves a number of problems to be resolved such as the problems concerning the concentration control of the solution, the control of the atmosphere for drying, the solution treatment cost during the drying process or the like. Therefore, in order to obtain an absorption belt capable of providing a satisfactory attraction, a high reliability and a high energy-saving ability, the above-mentioned method (3) may be more useful if improved further.
The present invention is directed to overcome one or more of the problems as set forth above.